To further increase data rates for mobile telecommunication, cellular networks use unoccupied radio resources in the unlicensed spectrum. Within the 3rd Generation Partnership Project (3GPP), the initiative for License Assisted Access (LAA) defines equipment for Long Term Evolution (LTE) and its next generation successors, which operates in the unlicensed radio spectrum, e.g., in frequency bands near 5 GHz. In LAA, the unlicensed spectrum is used as a complement to the licensed spectrum. Accordingly, stations (including a network node and a wireless device) that are radio-connected on a licensed carrier in the licensed spectrum (which is also referred to as a primary cell or PCell) use carrier aggregation to benefit from the additional transmission capacity of unlicensed carriers in the unlicensed spectrum (which are also referred to as a secondary cells or SCells). To reduce changes required for aggregating licensed and unlicensed spectrum, a radio frame timing defined according to LTE in the primary cell is simultaneously used in the secondary cells. Further initiatives such as MulteFire within the MulteFire Alliance define standalone implementations based on LTE relying exclusively on the unlicensed spectrum.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior sensing on the unlicensed carriers. Since the unlicensed spectrum must be shared with other radio transmitters of similar or dissimilar wireless technologies, a so-called listen-before-talk (LBT) process is applied. For example, the unlicensed 5 GHz spectrum can also be used by equipment implementing a Wireless Local Area Network (WLAN) according to the standard family IEEE 802.11, which is also known as Wi-Fi.
Wi-Fi as well as LAA are capable of operating in a multi-carrier mode, i.e., transmitting simultaneously on multiple unlicensed carriers, e.g., in the 5 GHz band. Conventional Wi-Fi follows a hierarchical multi-carrier LBT scheme, which is known as channel bonding.
In conventional LTE, licensed carriers can be aggregated and utilized for data transmission to boost the throughput. Due to the introduction of LAA in 3GPP Release 13, for example according to the document 3GPP TR 36.889 V13.0.0 (2015-06), there is a need to support multi-carrier operation on unlicensed carriers.
However, conventional LBT designs lead to an inefficient multi-carrier operation. As for one example, if a station is transmitting on one or more carriers, same station cannot listen on any further non-transmitting carrier during the transmission, e.g., by design or due to cross-carrier self-interference. The cross-carrier interference may relate to adjacent channels of the same station, which can leak energy to each other. Herein, “adjacent” includes (but is not limited to) channels that are seamlessly next to each other. As for same or another example, a station may transmit on one channel and receive on other channel, if the channels are located sufficiently apart in the spectrum. Therefore, for downlink data transmission, it is important to increase the likelihood that multiple carriers complete the LBT process and simultaneously start transmitting, since the network node may be unable to perform further channel sensing or data reception as long as it is transmitting.
For uplink data transmission, a scheduled radio access technology such as LTE requires the network node to transmit uplink scheduling grant information to the wireless device before the wireless device transmits its data. If the transmission of grant information on multiple carriers in the downlink is treated similarly to the downlink data transmission, the network node performs an LBT process on each carrier in the unlicensed spectrum in order to access the channel and grant the wireless device the uplink data transmission.
If the LBT succeeds on one carrier, the network node transmits the grant on this carrier. Only the carriers that transmit the grant can grant the wireless device an uplink transmission. The problem with this solution is that each carrier may end up with different intended transmit times, e.g., due to a random backoff of the LBT process on each carrier and different interference levels on each carrier. As a result, only one carrier is actually utilized for grant transmission and it is very unlikely to operate on multiple carriers neither for grant transmission nor for the uplink data transmission in unlicensed spectrum. Moreover, as the number of aggregated or available carriers increases, the probability that each of these carriers is utilized further decreases.
Since the time of successful conclusion of LBT processes performed on carriers in the unlicensed spectrum is uncertain and the downlink transmission on the first successfully accessed carrier defers any other LBT process, the number of carriers for which the wireless device receives an uplink scheduling grant from the network node is uncertain.